Bashir

You may be looking for Changeling, who impersonated Dr. Julian Bashir.

Dr. Julian Bashir is a 24th century Starfleet officer, known for serving as chief medical officer of Deep Space 9 under Benjamin Sisko. The presence of Bashir in Star Trek Timelines was first revealed in the An Update on Beta…and a Borg Cube dev blog where it is stated that there is a "Secret Agent" Bashir character.